The invention relates generally to a novel method and apparatus for heating material, and more particularly, to a method and apparatus for melting metal, particularly aluminum, wherein the rate of heat transfer to the melt is greater than that obtainable with prior known melting methods.
Conventional melting techniques for metal, particularly aluminum and aluminum scrap, utilize natural gas reverberatory furnaces wherein heat is transferred to the melt primarily by radiation. Such processes are inefficient with, at best, only 30% of the heat of combustion ending up as useful heat in the liquid metal.
It is also known that metal can be heated or melted in furnaces utilizing electric arcs as a heat generating means. U.S. Pat. No. 3,147,329 to Robert Gage describes two conventional electric arc heating methods. One method (referred to as the nontransferred method) comprises a device or torch having two electrodes which heats the material to be melted by exposing it to radiation emanating from an electric arc generated between the electrodes. Alternatively, the material may be heated by exposing it to a gas stream having been heated by the arc as it passes therethrough. The second method described by Gage is referred to as the transferred arc heating method. In this method, the melt itself acts as an electrode which permits the direct transfer of energy from the arc to the melt, thereby heating the melt by resistance heating.
Both the transferred and nontransferred methods are plagued with difficulties when melting metal, particularly aluminum. The nontransferred arc heating methods, such as that found in conventional plasma arc torches, suffer from low heat transfer efficiencies. The transferred arc heating methods, such as those found in arc furnaces, have problems with electrode consumption, furnace refractory damage, product contamination, hot spots, and more significantly, reduced product recovery or melt loss resulting from undesirable vaporization of the melt caused by the high temperatures and localized high currents associated with the arc discharge of these methods.
Aluminum is a difficult metal to melt because its relatively low melting temperature causes it to vaporize rather easily. Such vaporization is undesirable because vaporized aluminum is rather difficult to condense and recover. Scrap aluminum, which generally contains large quantities of aluminum beverage cans having ends made of high magnesium Aluminum Association alloy 5182, is particularly difficult to melt because the magnesium of AA 5182 tends to ignite easily and start fires in the melting furnace. Such fires are undesirable, quite obviously, because they can result in the loss of significant amounts of aluminum, thereby substantially reducing product recovery.
Accordingly, an object of the present invention is to provide a method and apparatus for heating material wherein material loss through vaporization and combustion is minimized.
Another object of the invention is to provide a method and apparatus for heating material having high heat transfer efficiency.
Yet another object of the invention is to provide a method and apparatus for melting recycled aluminum scrap wherein combustion of aluminum and magnesium contained in the scrap is minimized.
These, as well as other objectives, will become apparent from a reading of this disclosure and the claims and an inspection of the accompanying drawings appended hereto. In accordance with the above objectives, the present invention provides a method of heating electrically conductive material. The method includes the steps of heating a jet of gas or gaseous mixture, directing the heated gas jet so as to make contact with the material, and drawing a diffuse current through said gas jet to said material by seeding it with an additive having a low ionization potential so as to increase the rate at which the material is heated, preferably without vaporizing the material.
The apparatus of the present invention for heating electrically conductive material comprises means for heating gas(es), means for directing a jet of said gas(es) to the material, means for introducing an additive having a low ionization potential into the gas(es) for purposes of ionizing said gas(es), and means for drawing a diffuse current through the jet of ionized gas to the material.
Diffuse current, as used herein, refers to the relatively uniform flow of electrical current throughout the cross section of the aforementioned gas jet, the amount of current in accordance with the present invention being below that value that would result in arc breakdown voltage.